SET vs OTL

Aarif,

I don't think there really is any standard for "neutrality," or any other subjective quality. What I would consider neutral (neutral what? -- high frequencies, low frequency, etc.?). I am not familiar with the Audiovalve RKV that you like, so it is hard to say what it will be closest to in terms of tonal balance, or any other quality.

I don't think that the theoretical discussions are that helpful either. Sure, Atmasphere is a designer of OTL amps and honestly believes that they are inherently superior (transformers are FAR from perfect in their conversion of high voltage/low current to low voltage/high current output, and arguably SET transformers are WORSE than pushpull transformers). The other camp can point to theoretical disadvantages of OTL vs. SET (need to use multiple output tubes in parallel for high output and sufficiently low output impedance, need for a servo circuit or blocking capacitor to prevent passing high voltage DC).

Also, there is quite a bit of difference between specific models for each type, plus the different way each model may interact with the specific speaker it is used with.

So, as a very rough generalization, I would say that almost all of the OTLs I've listened to (Graaf, Fourier, SAP, Atmasphere, Joule, custom designs) were exremely fast, dynamic and exciting (the Tenor was the exception). Once you've had your ears pricked up by an OTL, it is hard to listen to more listless and dull amps. I liked the tonal balance, so I would call it "neutral," but, I could see how someone else might find certain of these amps a bit lean. The "excitement" also comes with a bit of rough and tumble quality that someone might describe as crude or "not refined" (someone who wants a smoother sound).

The SET amps I've heard vary FAR more than the OTL amps I've heard. The better amps deliver a pure, natural sound, dense and complete harmonic structure, and natural sounding attack and decay of notes (the notes seem to bloom into space like they would in a real hall). But, depending on the design and type of tube, one can get SET amps that sound lean and fast and dynamic (within their restricted power output), and others that sound warm, sluggish (smooth ?) and woolly. "Neutral" or not, there is something seductive about good SET amps that is harder to describe (you know it if you hear it), while the special qualities of good OTLs are pretty evident.

I would put the Cary 300 SE you mentioned well into the warmer, smooth side. If you think the Graaf is neutral in balance, the Cary may not be for you. I actually liked the Cary even though it is radically different sounding compared to my SET amp, but only after the tubes had been swapped (the stock amp can be substantially changed in sound by tube selection).

I only heard the Graaf briefly, but, I recall being favorably impressed. It sounded similar to the SAP OTL I heard and like quite a bit. You should also seriously consider the 30 watt Atmasphere OTL. A friend had that amp and I thought it sounded terrific (he traded it in for something else because of noise -- a bigger issue in his system than it would be in almost anyone else's system because his speakers are 105 db/w efficient).

By the way, I've never heard the WAVAC, but, I heard an amazingly good Audionote Sogon amp (211 tube) that cost something like $250,000. I know someone who has two pairs of those amps. Crazy world.

Paulfolbrecht, since you asked- in our amps there is only one stage of gain, not two. Most SETs I've seen have either two or three. This is because there is usually some gain in the output, while in our amps there is none.

Crossover distortion, FWIW, is not a function of the 'phase splitter', it is a function of the push-pull operation of the output section. A number of crossover issues stem from output transformers, and some of them stem from class of operation. In a class A push-pull amp, carefully designed, you will not get any crossover distortion at all.

You are right in that if there are crossover issues, they can often appear at low power only, masking low level detail. This is something that simply does not happen in our amps, and it is easy to hear how they have plenty of low level detail, actually more than SETs running at similar power levels do.

You are also right in that harmonic spectra is not the defining issue regarding sound of OTLs and SETs. I should point out here though that most OTLs employ negative feedback, often fairly large amounts, while hardly any SETs use feedback. Our OTLs use little (1 or 2db) or none, so in a way they tend to have more in common with SETs sonically than they they do with a lot of push-pull amps.

Larryi, just for the record, an OTL that has a direct-coupled output does not have to have either a coupling cap or a servo circuit. They can be built to be so stable that the DC Offset can always be very small and only need adjustment occasionally, once every 3-6 months.

The multiple power tube issue is shared by SETs with multiple tubes, Push-pull with multiple tubes and of course transistor amps with multiple output devices. In all cases, it is possible to build the amplifier so the multiple output devices are not a defining characteristic.

I'd strongly disagree with Paul's comments re: SET amps not sounding "lush" vs OTLs.

While my earlier comments re: the "romantic" sound of SET amps were primarily intended to describe 300B SETs (and more specifically the Cary monos I own), I find some elements of this "lush" quality in all of the SET amps I've heard. I would specifically include the Cary 805s I also own, and more specifically include the 805s when used with the 211 output tube.

I have never gone the "flea power" route, so I can't comment on those, but I've heard 300B based amps from several manufacturers and ALL of them sounded warm tonally, so it ain't just Cary. As to OTLs sounding "whitish", I've heard that, too. My early production S-30s could sound this way when paired with the wrong speaker.

Marty

Incidentally, nothing wrong in my book with "lush" sounding amps. That's precisely the reason I own two pairs of SETs.

Marty